Propylene is one of the fastest growing petrochemicals primarily because of the high growth rate of polypropylene. Studies show that the worldwide demand for propylene has been increasing at an annual average rate of 5.7% since 1991. In the year 2000, propylene production was about 52 million tones and it is projected that the demand will grow to 84 million tones by the year 2010. Typically, about 70% of this propylene is generated by steam cracker, 28% by refinery fluid catalytic cracking (FCC) units, and 2% by on-purpose processes like propane dehydrogenation or metathesis. The growth rate of the propylene demand has significantly outpaced the demand for ethylene growth rate, which is also produced from steam cracker. As a result, the construction of new steam crackers to meet the increasing ethylene demand alone or also will not be sufficient to satisfy the growing propylene demand. To make up for this shortfall, other propylene supply sources will be required. Therefore, additional emphasis is being given on recovering propylene from FCC units involving the addition of ZSM-5 catalyst and new technologies such as DCC (Deep Catalytic Cracking), high severity FCC riser cracking e.g. Indmax, PetroFCC or on-purpose processes such as propane dehydrogenation, metathesis olefinic naphtha cracking technologies (MOI, Superflex, Propylur or PCC). The capability of on-purpose processes is not enough to match the growth of propylene demand. This means that the market demand for propylene has to be met from other processes like FCC or new technology such as DCC (Deep Catalytic Cracking) or Indmax. In order to increase propylene yield through DCC, Indmax, Superflex or PCC technologies, refiners need to invest for setting up new units.
U.S. Pat. No. 6,977,321 describes a process for the production of propylene from cracking of olefinic feedstock on crystalline silicate catalyst comprising an MFI (Meet Flow Index) structure having silicon/aluminum ratio within the range of 180 to 1000. It is carried out at a temperature of 500 to 600° in two parallel swing reactors. It is capable of processing only lighter hydrocarbons. Higher silica/alumina ratio catalyst employed has lower activity which leads to fluctuation of product selectivity while it operates in swing reactor mode. U.S. Pat. No. 5,043,522 describes conversion of predominantly paraffinic feedstock on ZSM-5 zeolite catalyst to C2 to C3 olefins. In this process even at very high reaction temperature and very low reactor pressure per pass conversion is very low (30 to 40%). Moreover, the reactor configuration used for the process is not disclosed, which is very important for obtaining sustained yield and product selectivities. U.S. Pat. No. 6,222,087 describes a process for converting C4 to C7 paraffin and olefins to ethylene and propylene by using ZSM-5 catalyst and/or ZSM-11. This process is carried out only in dense fluidized bed reactor or fixed-bed swing reactor. The example described in this patent shows the formation of good amount of BTX (benzene, tolune, xylene) while processing butane-1 or the like feed due to predominant oligomization reaction. Further, propylene production and conversion are not high when LCN is processed. U.S. Pat. Nos. 5,043,522 and 5,171,921 describe a process for the production of C2-C5 olefins from higher olefinic or paraffinic or mixed olefins and paraffin feedstock over steam activated catalyst containing phosphorus and H-ZSM-5. As the coke yield is less than 0.5 wt % of the feed, heat necessary to maintain the reaction is to be provided by separately heating the catalyst particles in a fluidized regeneration zone for instance by combustion of appropriate fuel hydrocarbon. A main drawbacks of this process is that the catalyst deactivates very quickly while the fuel burns in the regenerator for supplying heat for the process. U.S. Pat. No. 6,951,968 discloses a process for converting the less valuable olefins present in refinery and petrochemical plants as a feedstock. It catalytically converts olefins into light olefins and in particular propylene, over an MFI (Melt Flow Index) crystalline silicate catalyst having silicon/aluminum aiomic ratio of 300 to 1000. The catalytic activity of this catalyst is very low as acid density is very low. The catalyst has a very high silica/alumina ratio as against catalysts used in typical FCC units where silica/alumina is in the range of 25 to 50. The process is mainly for use in moving bed reactor like catalytic reforming reactor where large quantity of heat needs to be supplied to maintain reaction temperature between 500 to 600° C. The above processes are essentially for making light olefins from C4 or higher hydrocarbon streams by using mainly ZSM-5 catalyst and fixed bed swing type or moving bed type reactor configuration. The reaction temperature is achieved by burning separate fuel. U.S. Pat. No. 7,323,099 describes a process for selectively producing C2 to C4 olefins from feedstock such as gas oil or resid. The feedstock is reacted in a first stage comprising a fluid catalytic cracking unit wherein it is converted in the presence of a mixture of conventional large pore zeolite catalyst and a medium pore zeolite catalyst to reaction products including naphtha boiling range stream. The naphtha boiling range stream is introduced into a second stage where it is contacted with a catalyst containing from about 10 to about 50 wt % of a crystalline zeolite having an average pore diameter less than about 0.7 nanometers at reaction conditions which include temperatures ranging from about 500 to about 650° C. and a hydrocarbon partial pressure from about 10 to about 40 psia. This process requires essentially two independent FCC units, wherein heavy feed cracked in the first riser in the presence of larger pore Y zeolite catalyst and medium pore zeolite like ZSM-5 and naphtha product from the first FCC unit, is further cracked in a second riser in the presence of a second catalyst containing medium pore zeolite catalyst mostly. Each of these risers has a lift zone where typically steam is used as lift medium. U.S. Pat. No. 4,830,728 discloses a FCC process that has two separate risers in which heavy feed/VGO (vacuum gas oil) cracked in first riser in the presence of catalyst mixture containing mainly large pore crystalline silicate zeolite and medium pore ZSM-5 type and ethylene rich material is introduced to a second riser at a lower level to produce heavier products in the presence of shape selective catalyst. Naphtha is also introduced into the second riser at a higher level thereby producing high octane gasoline. The lift zone of the second riser is used to carry out exothermic oligomerization reaction for converting ethylene to heavier products to maximize high octane gasoline. US 20,080,035,527 describes a dual riser FCC process for converting naphtha, mixed C4 stream or the like to ethylene and propylene in the presence of an FCC catalyst. This process requires coke precursor or auxiliary fuel to satisfy the heat balance of the unit for converting light hydrocarbon stream. It relates to cracking of light and heavy naphtha streams in different risers so that cracking severity can be adjusted separately in each riser depending on the cracking severity requirement. Butadiene is used to let down more coke on the catalyst in the riser or fuel gas or fuel oil is used in the regenerator to supply supplemental heat. US 20,060,108,261 describes a process for converting naphtha in FCC type configuration using ZSM-5 family catalyst. It also describes improvement in propylene making by recycling C4 fraction to a dilute phase reaction zone to separate the dense phase stripping zone. US 20, 0401,082,745 and WO 2,004,078,881 relate to sequential cracking of C6 lean and rich fraction in one or more fixed bed reactors for making propylene in the presence of medium pore zeolite and silico alumino phosphate. EP 1,555,308 discloses recycle of naphtha at the lift zone of a FCC unit riser. However, it emphasizes that naphtha cracking in separate risers is advantageous. The above processes in general teach conversion of olefinic naphtha feedstock or C4 or higher olefinic hydrocarbon feedstock to light olefin particularly propylene over MFI (Melt Flow Index) crystalline silicate catalyst in fluid bed or dense bed or fixed bed with swing reactor or dual riser system. Heat balance is satisfied by using supplementary fuel supply. It is also known in the prior art processes to recycle light olefinic naphtha at the riser bottom for increasing C2 to C4 olefins irrespective of the preferred length of lift zone which is to provide optimum vapour residence time, weight hourly space velocity or the like. The lift steam is used to keep the catalyst above choking velocity. However, lift steam causes deactivation and attrition of the catalyst as regenerated catalyst comes in contact with steam at very temperature in the range of 690 to 740° C. Steam also increases water generation in the reaction. Fluid catalytic cracking technology is used in refineries to crack light olefin rich hydrocarbons stock with naphtha. A typical FCC unit comprises at least one riser having an acceleration zone or lift zone at the lower portion thereof, a lift stream feed nozzle at the bottom thereof and a light olefinic hydrocarbon stock feed nozzle above the lift stream feed nozzle in spaced apart relationship. The riser optionally comprises an olefinic naptha feed nozzle at a location along the acceleration zone. A lift stream comprising lift steam or inert lift flue gases like refinery fuel gas or combination thereof is introduced through the lift stream feed nozzle at the bottom of the riser. An olefinic rich hydrocarbon stock is introduced into the riser through the hydrocarbon stock feed nozzle. The catalyst is fed into the riser bottom from the regenerator. Naptha is optionally fed into the riser along with the lift stream or through the naptha feed nozzle. Catalytic cracking of the hydrocarbon stock and naptha, if any, take place in the riser. (Fluid Catalytic Cracking Handbook Design, Operation, and Troubleshooting of FCC Facilities by Reza Sadeghbeigi, Gulf Publishing Company, Houston. Tex., 1995)
It is advantageous and desirable to increase the yield of propylene and ethylene obtained by fluid catalytic cracking (FCC) process in a given FCC unit without increasing the capacity of the FCC unit or without any hardware alterations in the FCC unit, reduce hydrothermal deactivation and attrition of the catalyst and water formation during production of propylene and ethylene in the FCC unit, crack the hydrocarbon feed stock at appropriate severity to maximize yields of diesel, gasoline, LPG (liquefied petroleum gas), propylene, ethylene or combination thereof in the FCC unit and use only olefinic C4 hydrocarbon in the lift stream to improve the equilibrium catalyst activity by at least 5 wt % for constant catalyst make up rate. There is, therefore, a need for realizing such advantages and benefits.